docs: def plot_depth_histogram(
          catalogue, bin_width,
          normalisation=False, bootstrap=None, filename=None,
          figure_size=(8, 6), filetype='png', dpi=300, ax=None):
      """
      Creates a histogram of the depths in the catalogue
  
      :param catalogue:
          Earthquake catalogue as instance of :class:
          openquake.hmtk.seismicity.catalogue.Catalogue
      :param float bin_width:
          Width of the histogram for the depth bins
      :param bool normalisation:
          Normalise the histogram to give output as PMF (True) or count (False)
      :param int bootstrap:
          To sample depth uncertainty choose number of samples
      """
      if ax is None:
          fig, ax = plt.subplots(figsize=figure_size)
      else:
          fig = ax.get_figure()
      # Create depth range
      if len(catalogue.data['depth']) == 0:  # pylint: disable=len-as-condition
          raise ValueError('No depths reported in catalogue!')
      depth_bins = np.arange(0.,
                             np.max(catalogue.data['depth']) + bin_width,
                             bin_width)
      depth_hist = catalogue.get_depth_distribution(depth_bins,
                                                    normalisation,
                                                    bootstrap)
      ax.bar(depth_bins[:-1],
             depth_hist,
             width=0.95 * bin_width,
             edgecolor='k')
      ax.set_xlabel('Depth (km)')
      if normalisation:
          ax.set_ylabel('Probability Mass Function')
      else:
          ax.set_ylabel('Count')
      ax.set_title('Depth Histogram')
  
      _save_image(fig, filename, filetype, dpi)